Pressure sensor having a compressible element

ABSTRACT

The invention relates to a pressure sensor ( 1, 41 ) comprising a chamber ( 3 ) in which a sensor element ( 4 ) for measuring pressure is arranged. The pressure sensor ( 1, 41 ) further comprises a compressible element ( 6, 7 ) for overpressure protection, said compressible element ( 6, 7 ) being arranged outside the chamber ( 3 ).

The invention specifies a pressure sensor which is in the form forexample of a differential pressure sensor, relative pressure sensor orabsolute pressure sensor. The pressure sensor is preferably filled witha fluid medium, for example oil.

Differential pressure sensors are described in the documents U.S. Pat.No. 4,543,832 A and U.S. Pat. No. 4,218,925 A.

Pressure sensors in which a compressible material is provided aredescribed in the documents DE 100 23 589 A1 and U.S. Pat. No. 5,792,958A.

An object to be achieved is that of specifying a pressure sensor whichis protected as well as possible against an overload.

In a first aspect, the invention specifies a pressure sensor which has achamber, in particular a chamber which is closed off from the outside.Preferably, a sensor element is arranged in the chamber.

Preferably, the chamber is formed at least partially by a housing of thepressure sensor. For example, the pressure sensor is designed to measurea pressure which is applied to an outer side of the chamber. The chamberis preferably filled with a fluid medium. For example, the chamber isfilled with oil. Preferably, the fluid medium is designed transmit apressure onto a sensor element, in particular to transmit a pressureapplied externally to the chamber onto a sensor element arranged in thechamber. In the following text, the fluid medium is also known astransmission medium.

Preferably, the pressure sensor is designed to measure a pressure of afurther fluid medium which exerts a pressure on the outer side of thechamber. In the following text, this further fluid medium is also knownas pressure medium. For example, the pressure medium exerts a pressureon a pressure-sensitive region which closes off the chamber from theoutside. A pressure-sensitive region is preferably a region which reactsto pressure fluctuations and which for example deforms in the event of apressure fluctuation.

For example, the pressure-sensitive region is in the form of a membrane,in particular a membrane which bounds or closes off the chamber. Thepressure medium may in particular be such that when the pressure mediumfreezes, a volumetric expansion of the pressure medium occurs. Forexample, the pressure medium includes urea.

Furthermore, the pressure sensor has a compressible element. Thecompressible element serves preferably to protect the pressure sensoragainst an overload, in particular excess pressure. The compressibleelement is preferably formed such that it decreases its volume in theevent of a rise in pressure. In particular, the compressibility of thecompressible element may be selected such that a considerable reductionin volume occurs as soon as the pressure acting on the compressibleelement is above a limit pressure.

For example, such a high pressure can occur during a phase transition ofa medium from a liquid to a solid phase, for example when a mediumfreezes. The phase transition can occur for example in the pressuremedium. Alternatively or in addition, the phase transition can occur inthe transmission medium. Furthermore, the limit pressure can also beexceeded without a medium freezing.

Thus, the compressible element is preferably designed such that itdecreases its volume, for example when a limit pressure is exceeded. Thechange in volume does not have to occur during a phase transition of themedium from a liquid to a solid phase. Alternatively or in addition, thecompressible element may be designed such that it decreases its volumeduring a phase transition of the medium from a liquid to a solid phase.

Preferably, the change in volume of the compressible element isreversible. In particular, the compressible element may include anelastic material.

The compressibility of the compressible element is preferably selectedsuch that, if the pressure rises above the limit pressure, it iscompressed to such an extent that no overloading of the pressure sensor,in particular no overloading of a sensor element arranged in thechamber, occurs. For example, the compressibility is such that acompression of the compressible element takes place during a phasetransition of a medium from a liquid to a solid phase, for example whena medium freezes. Preferably, the compressible element absorbs theincrease in volume of the fluid medium during the phase transition, forexample during freezing. In this way, it is possible to prevent thepressure sensor, and in particular a sensor element of the pressuresensor, being damaged.

Preferably, the pressure sensor has a pressure line for supplying apressure to be measured to an outer side of the chamber. By way of thepressure line, a pressure to be measured is guided preferably to apressure-sensitive region. For example, the pressure line is designed tosupply a pressure medium from the outside onto a membrane bounding thechamber.

In the event of a rise in the external pressure, in particular apressure of the pressure medium, the volume of the chamber preferablydecreases. For example, a deformation of a membrane which closes off thechamber from the outside occurs. On account of the decrease in volume,the pressure inside the chamber increases until the internal pressure isthe same as the external pressure. Preferably, a sensor element by wayof which the internal pressure and thus also the external pressure canbe determined is located inside the chamber.

In one embodiment, the compressible element is arranged in theclosed-off chamber.

In this case, a compression of the compressible element takes placepreferably at a sufficiently high internal pressure. In the event of acompression of the compressible element, the internal pressure islowered again. This lowering of the internal pressure with respect tothe external pressure is compensated by a further decrease in thechamber volume, until the internal pressure is the same as the externalpressure again.

Preferably, the compressibility of the compressible element is selectedsuch that only slight compression of the element occurs at a nominalpressure of the pressure sensor.

Preferably, the pressure sensor is designed such that the volume of thechamber can be decreased only to a minimum volume. Correspondingly, theinternal pressure prevailing in the chamber can rise only to a maximumpressure.

Preferably, the pressure sensor has a device which prevents the volumedecreasing below the minimum volume.

For example, the pressure sensor has a stop for a membrane closing offthe chamber. In this case, the membrane can be deformed by an externalpressure acting on the membrane only until it bears against the stop.Thus, the minimum pressure in the chamber and in particular the maximumpressure prevailing in the chamber are defined by the stop. Inparticular, when the membrane bears against the stop, the internalpressure is independent of the external pressure and is at apredetermined maximum value.

Preferably, as a result of the compressible element, in particular whenthe compressible element is arranged inside the chamber, the maximumpressure is lower than in the case of a pressure sensor which isconfigured in the same way and has no compressible element. Thus, thepressure sensor, and in particular a sensor element arranged in thechamber, can be protected against an overload.

If the external pressure is still below the maximum pressure, anincrease in the external pressure results preferably in a reduction inthe volume of the chamber and thus to an increase in the internalpressure. This results in compression of the compressible element. Onaccount of the compression of the compressible element, the internalpressure is lowered such that, on account of the external pressureapplied, the volume of the chamber is decreased further. If the externalpressure is higher than or the same as the maximum pressure of thepressure sensor, the volume of the chamber is decreased until theminimum volume and thus the maximum pressure in the chamber has beenreached. A further decrease in volume is prevented for example by themembrane bearing against a stop. Thus, on account of the compressibleelement, the minimum volume of the chamber is taken up at a lowerpressure. Thus, the internal pressure can rise only to a maximum valuewhich is determined by the compressibility of the element. In the eventof a further increase in the external pressure, the volume of thechamber cannot decrease further and thus the pressure in the chambercannot rise above the maximum pressure.

In a further embodiment, the compressible element is arranged outsidethe closed-off chamber.

Particularly preferably, the chamber is in this case closed off from theoutside by a pressure-sensitive region. Preferably, thepressure-sensitive region is in the form of a membrane, in particular ametallic membrane.

The pressure sensor preferably has a pressure line for directing thepressure of a fluid medium, in particular the pressure of a pressuremedium, onto the pressure-sensitive region. The pressure line ispreferably fillable with the fluid medium. The compressible element ispreferably arranged on the pressure line.

With such a configuration of the pressure sensor, the pressure sensorcan be protected particularly well against an overload by freezing ofthe pressure medium. Preferably, the compressible element absorbs avolumetric expansion of the pressure medium. In this way, in particulardamage to the pressure-sensitive region, for example damage to ametallic membrane, is prevented.

In a further embodiment, compressible elements may be provided bothinside and outside a chamber.

Preferably, a sensor element having a pressure-sensitive region isarranged in the chamber.

The pressure-sensitive region is designed for example such that in theevent of applied pressure, deformation of the region occurs and as aresult a pressure value can be determined. For example, the pressuresensor is in the form of an oil-filled piezoresistive pressure sensor.Ceramic pressure sensors having a resistive or capacitive operatingprinciple are likewise possible.

The chamber is preferably closed off from the outside by a membrane.

The membrane separates in particular the internal region of the chamberfrom an external region. If a transmission medium is present in thechamber and a pressure medium is present outside the chamber, then themembrane preferably separates the media from one another. The membraneis preferably hermetically sealed. The membrane serves in particular toprotect a sensor element arranged in the chamber. For example, themembrane prevents an aggressive pressure medium from coming into contactwith the sensor element. The membrane is preferably a metallic membrane.For example, the membrane includes steel, in particular stainless steel.

Preferably, a pressure medium acts from the outside on the membrane.Preferably, in the event of a rise in pressure, the membrane is deformedsuch that the volume in the chamber is decreased. Thus, the externalpressure is transmitted into the interior of the chamber.

By way of the membrane, it is possible to prevent a medium from flowinginto the chamber from the outside, and so the chamber can be protectedagainst being overloaded by a medium flowing in. Preferably, compressionof the compressible element that occurs at a nominal pressure of thepressure sensor is compensated by a deformation of the membrane, suchthat no distortion of the pressure measurement by the compressibleelement occurs.

In one embodiment, the housing of the pressure sensor has a stop forlimiting the deflection of the membrane.

The stop is formed for example by one or more surfaces against which themembrane bears when a maximum permissible external pressure has beenreached. In the event of a further increase in pressure, the membranecannot deform further on account of the stop, and so the pressure in theinterior of the chamber cannot rise further.

For example, the chamber has a pressure line for directing an externalpressure onto a sensor element. The pressure line preferably has asmaller diameter than a region of the chamber in which the sensorelement is arranged. Preferably, the membrane is located at that end ofthe pressure line that is remote from the chamber.

The fluid medium in the chamber is preferably an incompressible liquid,for example oil. If the pressure sensor is in the form of a differentialpressure sensor, two chambers filled with a fluid medium may beprovided. Preferably, the same fluid medium is located in both chambers.

For example, the pressure sensor can have only one compressible elementor a plurality of compressible elements.

In one embodiment of the pressure sensor, in which the compressibleelements are arranged inside the chamber, the compressible elements maybe arranged in a distributed manner in the chamber. In one embodiment,the compressible element lines the inner wall of the chamber. In afurther embodiment, the compressible element is formed in a spherical orapproximately spherical manner. Preferably, the compressible element isin this case surrounded as completely as possible by the fluid medium.In this case, particularly uniform and rapid compression of thecompressible element is possible.

In one embodiment of the pressure sensor, in which the compressibleelement is arranged outside the chamber, the compressible element isformed for example in a cylindrical manner. Preferably, a pressure lineis formed as a feedthrough through the compressible element. Preferably,the compressible element has a shape that is producible as easily aspossible, for example a shape which is producible in an extrusionprocess.

Preferably, the total volume in the case of a compressible elementarranged inside the chamber is at least 50% of the volume of the fluidmedium in the chamber.

Preferably, the compressible element is arranged as close as possible toa pressure-sensitive region of the pressure sensor. This allowsparticularly effective protection of the pressure sensor against damage,for example when the fluid medium freezes.

In a preferred embodiment, the distance of the compressible element froma pressure-sensitive region is less than or equal to 1000 μm,particularly preferably less than or equal to 500 μm.

In this case, the distance of the compressible element from thepressure-sensitive region is the minimum distance of the sensor elementor of the pressure-sensitive region from a compressible element.

The invention furthermore specifies a pressure sensor which has twochambers which are closed off from the outside and are each filled witha fluid medium. Such a pressure sensor serves for example as adifferential pressure sensor. At least one compressible element may bearranged in each chamber. As an alternative or in addition thereto, acompressible element may be arranged outside the chambers, in particularin two pressure lines.

In a further aspect, the invention specifies a sensor element for apressure sensor.

For example, the sensor element is used in the case of a pressuresensor, in which the sensor element is arranged in an outwardly openregion, for example an open pressure feed line, of the pressure sensor,such that a fluid medium can flow in and out.

The sensor element may also be used in other embodiments of pressuresensors. The sensor element is used for example in the above-describedpressure sensor and is in this case arranged in the chamber of thepressure sensor.

The sensor element has a pressure-sensitive region for measuring apressure of a fluid medium.

Preferably, the sensor element has a compressible element for protectingthe sensor element against an overload. The compressible element iscompressed preferably at a pressure above the nominal pressure of thesensor element such that it absorbs the pressure overload and thusprotects the sensor element against damage.

Alternatively or in addition, in the event of a phase transition of thefluid medium from liquid to solid, the fluid medium is prevented fromflowing in. Thus, the rise in pressure at the sensor element iseffectively limited by the phase transition of the medium and thecompressible element.

The compressible element may have all of the properties of thecompressible element described in connection with the pressure sensor.The compressible element of the sensor element may be present inaddition to further compressible elements arranged in the chamber. Inone embodiment of the pressure sensor, only the sensor element has oneor more compressible elements, while the chamber is free of furthercompressible elements.

In the following text, further configurations of the above-describedpressure sensor and of the sensor element are described. The pressuresensor and/or the sensor element have a pressure-sensitive region. Thepressure-sensitive region may be formed for example by a region of asensor element, in particular by a piezoresistive membrane.Alternatively, the pressure-sensitive region may be in the form of aregion which bounds a chamber, wherein a sensor element is located inthe chamber. For example, in this case the pressure-sensitive region isin the form of a metallic membrane.

Preferably, the distance of the compressible element from thepressure-sensitive region is as small as possible. For example, thecompressible element is spaced apart from the pressure-sensitive regionto such an extent that the pressure measurement is not impeded. Forexample, the pressure-sensitive region is deformable, for example in theform of a membrane. In this case, a deformation of thepressure-sensitive region by the compressible element should not beimpeded.

Preferably, the distance of the compressible element from thepressure-sensitive region is less than or equal to 1000 μm, particularlypreferably less than or equal to 500 μm.

In one embodiment, the pressure sensor or the sensor element has apressure line for directing the pressure of a fluid medium onto thepressure-sensitive region. The pressure line is fillable with a fluidmedium and filled with the fluid medium during a pressure measurement.

Preferably, the compressible element directly adjoins the pressure line.For example, the compressible element forms the lateral boundary of thepressure line. In this case, the fluid medium in the pressure line canexpand laterally in the event of freezing, wherein the compressibleelement is compressed and in this way overloading is prevented.

The pressure sensor or the sensor element may have a housing. In thiscase, the pressure line extends preferably in the housing.

Preferably, the pressure-sensitive region of the housing is connectedfirmly. The housing may form a support for the pressure-sensitiveregion. For example, the pressure-sensitive region is in the form of amembrane which is supported by the housing.

Preferably, the housing has an inner wall which is lined with thecompressible element.

The compressible element fills for example the entire interior space ofthe housing, apart from the pressure line. The pressure line leadspreferably through the compressible element.

For example, the pressure line extends vertically toward thepressure-sensitive region. In a further embodiment, the pressure linecan lead at an inclination angle, for example at an angle of 45°, towardthe pressure-sensitive region.

The pressure line may also have sections which have different diameters.In the case of such a pressure line, the behavior of the pressure sensorduring a phase transition, for example when the medium freezes, can beset in a particularly flexible manner. As a result, the effect of afreezing medium on the pressure-sensitive region can be decreased.

For example, the total volume of all of the compressible elements of thepressure sensor or of the sensor element is at least 50% of the volumeof the pressure line.

Given such a volume ratio, when the medium freezes, the resulting volumecan be absorbed by compression of the compressible element.

In the following text, the subjects described here are explained in moredetail with reference to schematic exemplary embodiments which are notto scale.

IN THE FIGURES

FIG. 1A shows a sectional view of a first embodiment of a pressuresensor at an external pressure less than the maximum pressure of thepressure sensor,

FIG. 1B shows the pressure sensor from FIG. 1A at an external pressuregreater than or equal to the maximum pressure of the pressure sensor,

FIG. 2 shows a sectional view of a second embodiment of a pressuresensor,

FIG. 3 shows a sectional view of a further embodiment of a pressuresensor having a compressible element,

FIG. 4 shows a sectional view of a further embodiment of a pressuresensor having a compressible element,

FIG. 5 shows a sectional view of a further embodiment of a pressuresensor having a compressible element,

FIG. 6 shows a sectional view of a possible configuration of thepressure sensor shown in FIG. 3.

FIG. 1A shows a pressure sensor 1 which is in the form of an absolutepressure sensor. The pressure sensor 1 has a housing 2 in which achamber 3 is formed. The chamber 3 is filled with a fluid medium 5. Thefluid medium 5 is preferably an incompressible liquid, for example anoil, in particular silicone oil.

Arranged in the chamber 3 is a sensor element 4. The sensor element 4has a piezoresistive membrane. When a force acts on the membrane, themembrane deforms, as a result of which resistances of a Wheatstonebridge change, making it possible to determine the pressure in thechamber 3. For example, the membrane is produced by removal of materialfrom a substrate, for example by means of etching.

The chamber 3 has a pressure line 10 which is filled with the fluidmedium 15. The pressure line 10 is closed off from the outside by amembrane 8 such that the chamber 3 having the pressure line 10 has aclosed-off liquid volume. When an external pressure greater than theinternal pressure in the chamber 3 is applied, the membrane 8 deformsand exerts a force on the fluid medium 5 in the chamber 3. The fluidmedium 5 is in the form of a transmission medium which transmits thepressure to the sensor element 4. Thus, a force is exerted on the sensorelement 4, by means of which the external pressure can be determined

The membrane 8, which closes off the pressure line 10 from the outside,is fastened to a part of the housing 10 and arranged above a stop 9 onthe housing 10. Preferably, the membrane 8 is a metallic membrane, inparticular a metal membrane. The housing also has a housing passage 22,which is in the form of a pressure line. Preferably, the housing passage22 is designed to direct a pressure medium 20 from outside onto themembrane 8. Preferably, the pressure sensor 1 serves to determine thepressure of the pressure medium 20.

In the case of an external pressure which is lower than the maximumpressure of the pressure sensor 1, the membrane 8 is spaced apart fromthe stop 9 and can deform in the direction of the pressure line 10. FIG.1A shows the pressure sensor 1 at such an external pressure.

If the external pressure exceeds the maximum pressure of the pressuresensor 1, the membrane 8 deforms in the direction of the stop 9 until itbears against the stop 9, as is shown in FIG. 1B. If the externalpressure increases further, the membrane 8 cannot deform further. Inthis way, damage to the membrane 8 by excessive deformation and anexcessive rise in pressure in the pressure line 10 and the chamber 3 isprevented. In addition, a minimum volume of the chamber 3 and thus amaximum pressure in the chamber 3 are defined by the stop 9. Thus,damage to the pressure sensor 1 and in particular to the sensor element4 at an external pressure greater than the maximum pressure isprevented.

As a result of the arrangement of the compressible elements 6, 7 insidethe chamber 3, the maximum pressure inside the chamber 3 is decreasedcompared with a pressure sensor which does not have the compressibleelements 6, 7. In this way, damage to the sensor element 4 can beprevented.

In a further embodiment, the pressure sensor 1 can be designed such thatdamage caused by an increase in pressure in the pressure chamber 3, forexample when the fluid medium 15 freezes, is prevented by thecompressible elements 6, 7.

The compressibility of the compressible elements 6, 7, i.e. the changein the volume of the elements 6, 7 per pressure change, is greater thanthe compressibility of the fluid medium 3. In particular, thecompressibility of the compressible elements 6, 7 is such that thecompressible elements are compressed in the event of a rise in pressurein the chamber 3 and decrease their volume. In this way, excess pressurein the chamber 3 is prevented. For example, the compressibility of thematerial of the compressible elements 6, 7 lies at Shore A values ofbetween 5 and 50.

FIG. 1B shows the pressure sensor 1 in a state in which the externalpressure is greater than or equal to the maximum pressure of thepressure sensor 1. The compressible elements 6, 7 have a reduced volumecompared with FIG. 1A, in which the external pressure is lower than themaximum pressure. The membrane 8 bears against the stop 9 such that thevolume of the chamber 3 cannot be reduced further. Thus, the internalpressure in the chamber 3 is equal to the maximum pressure. Even in theevent of an increase in the external pressure, the internal pressuredoes not rise further. Thus, when the membrane 8 bears against the stop9, the internal pressure is decoupled from the external pressure.

The compressible elements 6, 7 are formed for example from a foam-likeor rubber-like material. The compressible elements 6, 7 are formed forexample in a spherical manner or have for example a shape having planarexternal surfaces, which approaches a spherical shape, for example theshape of a dodecahedron. The compressible elements 6, 7 are preferablycompletely surrounded by the fluid medium 15.

In this case, in the event of a rise in pressure, a force acts from allsides on the compressible elements 6, 7, such that particularly rapidand uniform compression can occur.

In further embodiments, differently shaped compressible elements 6, 7,for example compressible elements which line the wall region of thechamber 3, may be arranged in the chamber 3. The number of compressibleelements may also vary, for example only one compressible element ormore than two compressible elements may be arranged in the chamber 3.

FIG. 2 shows a further embodiment of a pressure sensor 1, wherein thepressure sensor 1 is in the form of a differential pressure sensor. Thepressure sensor 1 has two chambers 31, 32 which are each filled with afluid medium 51, 52. Arranged in each chamber are compressible elements61, 71 and 62, 72, respectively, which protect the pressure sensor 1against excess pressure. Arranged between the chambers 31, 32 is asensor element 4 which measures the pressure difference between thechambers 31, 32. The chambers 31, 32 each have a pressure line 11, 12which leads to a membrane 81, 82 which is acted upon by an externalpressure. The external pressure is exerted in each case by a pressuremedium 201, 202, which is guided from outside onto the membrane 81, 82through a housing passage 221, 222. The differential pressure sensor 4thus measures the difference between the external pressures, inparticular the difference between the pressures present in the pressuremedia 201, 202.

In terms of its details, the pressure sensor 1 may be designed in amanner corresponding to the pressure sensor 1 shown in FIGS. 1A and 1B.

FIGS. 3, 4 and 5 show further embodiments of a pressure sensor 41 havinga compressible element 6. The compressible element 6 is arrangedpreferably as close as possible to a pressure-sensitive region 17 of asensor region 21 of the pressure sensor 41.

The pressure sensor 41 may be designed for example in a similar mannerto the pressure sensors 1 shown in FIGS. 1A, 1B and 2. In particular,the sensor region 21 may be formed like the chamber 3 in FIG. 1A and thepressure-sensitive region may correspond to the membrane 8 from FIG. 1A.This embodiment is described in detail in FIG. 6.

As an alternative thereto, the sensor region 21 may be designed forexample like the sensor element 4 from FIG. 1A. The pressure-sensitiveregion 17 is in this case formed for example as a piezoresistivemembrane of the sensor element 4. The compressible element 6 is arrangedpreferably as close as possible to the pressure-sensitive region 17 ofthe pressure sensor 41, such that it does not impede the deformation ofthe pressure-sensitive region 17 in the event of applied pressure.

The pressure sensor 41 has a sensor housing 16 which forms a housingpassage 22. The housing passage 22 is in the form of a tubular section,at one end of which the pressure-sensitive region 17 of the pressuresensor 41 is located. The housing passage 22 allows in particular afluid medium 15 to be supplied to the pressure-sensitive region 17.

The inner walls of the housing 16 are lined with the compressibleelement 6. In this way, a pressure line 13 which has a narrower crosssection compared with the housing passage 22 bounded by the housing 16is formed. The pressure line 13 extends at least partially through thecompressible element 6. In particular, the compressible element 6 isdesigned such that the region of the pressure line 13 which has thesmallest diameter is bounded by the compressible element 6. The pressureline 13 has in particular no constriction which is bounded by thehousing 16.

Preferably, the compressible element 6 has a shape which is as easy toproduce as possible. For example, the compressible element 6 has a shapewhich is translationally invariant in one direction, for example acylindrical shape. Such a compressible element 6 may be produced forexample in an extrusion process. In particular, in an extrusion processa strip can first be formed which is then divided into a plurality ofcompressible elements 6.

The housing 16 is for example in the form of a support on which apressure-sensitive region 17 configured as a membrane rests. Preferably,the membrane is a metallic membrane. The membrane can be held at itslateral ends by the housing 16.

In an alternative embodiment, the membrane is for example a siliconmembrane. For example, the membrane is formed by etching away materialfrom the carrier substrate. For example, the region inside the housing,in which the pressure line 13 and the compressible element 6 arearranged, is formed by removal of material during the production of themembrane. For example, the membrane is formed in one piece with thehousing 16.

In FIG. 3, the pressure line 13 extends vertically toward thepressure-sensitive region 17 of the pressure sensor 41.

During the pressure measurement, the pressure line 13 is filled with afluid medium 15. For example, the fluid medium 15 is a pressure medium,the pressure of which is intended to be determined by the pressuresensor 41. In particular, the fluid medium 15 may be formed like themedium 20 from FIG. 1A. Alternatively, the fluid medium 15 may be in theform of a transmission medium which transmits an external pressure ontoa sensor element. In this case, the fluid medium 15 is formed forexample like the medium 5 from FIG. 1A.

In the event of the fluid medium 15 freezing in the pressure line 13,the volume of the compressible element 6 is decreased such that anincrease in the volume of the fluid medium 15 is compensated by areduction in the volume of the compressible element 15. In this way, thepressure-sensitive region 17 is protected against excess pressure.

FIG. 4 shows a further embodiment of a pressure sensor 41 having acompressible element 6 arranged preferably as close as possible to thepressure-sensitive region 17. In this embodiment, the pressure line 13extends diagonally in the housing 16 and at an inclination angle, inthis case about 45°, to the pressure-sensitive region 17.

FIG. 5 shows a further embodiment of a pressure sensor 41 having acompressible element 6 arranged preferably as close as possible to thepressure-sensitive region 17. In this embodiment, the pressure line 13has sections 18, 19 with different diameters.

FIG. 6 shows a possible embodiment of the pressure sensor 41 from FIG. 3in detail. The sensor region 21 is in the form of a closed-off chamber 3which is filled with a fluid medium 5, in particular oil. The chamber 3is bounded by a membrane 8. The membrane 8 is preferably a metallicmembrane. A pressure acting on the membrane 8 results in a deformationof the membrane 8 and thus a decrease in the volume of the chamber.Preferably, the membrane 8 is designed such that locally arisingpressure peaks are averaged. Thus, pressure peaks in the chamber 3 areavoided and damage to a sensor element 4 arranged in the chamber 3 isprevented.

As a result of the compressible element 6, the membrane 17 is protectedagainst an overload. In particular, when the pressure medium 20 freezes,the compressible element 6 prevents damage to the membrane 17 occurringas a result of a volumetric expansion of the pressure medium 20.

The compressible element 6 is designed such that, when the fluid medium15 located in the pressure line 13 freezes, damage to the membrane 8 isprevented.

LIST OF REFERENCE SIGNS

-   1 Pressure sensor-   2 Housing-   3 Chamber-   31, 32 Chambers-   4 Sensor element-   41 Pressure sensor-   5 Medium-   51, 52 Fluid media-   6, 7 Compressible element-   61, 62, 71, 72 Compressible elements-   8 Membrane-   81, 82 Membranes-   9 Stop-   91, 92 Stops-   10 Pressure line-   11, 12, 13 Pressure lines-   15 Fluid medium-   16 Housing of the sensor element-   17 Pressure-sensitive region-   18, 19 Sections-   20 Pressure medium-   201, 202 Pressure media-   21 Sensor region-   22 Housing passage-   221, 222 Housing passages

The invention claimed is:
 1. A pressure sensor, comprising: a chamber inwhich a sensor element for measuring a pressure is arranged; and acompressible element for protecting against excess pressure, wherein thecompressible element is arranged outside the chamber, wherein thecompressible element is at least partially surrounded by a fluid medium,wherein the compressible element is adapted to decrease its volume whenthe pressure in the fluid medium rises above a limit pressure, andwherein the volume is taken up by the fluid medium, leading to adecrease of pressure in the fluid medium.
 2. The pressure sensoraccording to claim 1, wherein the chamber is bounded by apressure-sensitive region.
 3. The pressure sensor according to claim 1or 2, further comprising: a pressure line, which is fillable with thefluid medium, for directing the pressure of the medium onto thepressure-sensitive region, wherein the compressible element is arrangedon the pressure line.
 4. The pressure sensor according to claim 1,wherein the pressure-sensitive region is in the form of a membrane. 5.The pressure sensor according to claim 4, wherein the membrane is ametallic membrane.
 6. The pressure sensor according to claim 1, furthercomprising: a housing, wherein the pressure line extends in the housing.7. The pressure sensor according to claim 6, wherein the housing has aninner wall which is lined with the compressible element.
 8. The pressuresensor according to claim 1, wherein the pressure line leads through thecompressible element.
 9. The pressure sensor according to claim 1,wherein the pressure line extends vertically toward thepressure-sensitive region.
 10. The pressure sensor according to claim 1,wherein the pressure line extends at an inclination angle toward thepressure-sensitive region.
 11. The pressure sensor according to claim 1,wherein the pressure line has sections which have different diameters.12. The pressure sensor according to claim 11, wherein the sectionhaving the smallest diameter is bounded by the compressible element. 13.The pressure sensor according to claim 1, wherein the total volume ofthe compressible elements is at least 50% of the volume of the pressureline.
 14. The pressure sensor according to claim 1, wherein the distanceof the compressible element from the pressure-sensitive region is lessthan or equal to 1000 μm.
 15. The pressure sensor according to claim 1,wherein the chamber is oil-filled.
 16. The pressure sensor according toclaim 1, wherein the compressible element is formed from a foam-like orrubber-like material.
 17. The pressure sensor according to claim 1,wherein the chamber is closed by a membrane and the compressible elementis located on a different side of the membrane than the sensor element.